University of Maryland Extension

Phosphorus deficiency - Vegetables

phosphorus deficiency

Phosphorus Deficiency Symptoms

Older tomato leaves will turn purple, especially on the undersides. This is caused by a deficiency of phosphorus and is most noticeable early in the season when a combination of undeveloped root systems and cool soil limits the amount of phosphorus taken up by affected plants. Applying a balanced soluble fertilizer to foliage or around the base of each plant will alleviate the condition.

Purple Leaves on Vegetable Seedlings and Transplants

Purple coloration of leaves is frequently an indication that sufficient phosphorus is not available to plant roots. This is a common problem in early spring because root systems are immature and nutrients from soil organic matter are not available to plants. Cool soil temperatures inhibit the activity of soil microorganisms and processes that cause nutrients to be released and made available to plant roots. This occurs to some degree on all soils but is more prevalent in soils with low fertility and organic matter content. The solution is to apply a balanced soluble fertilizer to the root zone (around the base of each plant) or to the leaves (be sure that to thoroughly spray leaf undersides). The best time for foliar applications is early morning or early evening when the stoma (leaf breathing pores) are open. Compost tea, seaweed extracts, fish emulsion, and store-bought soluble fertilizers are all suitable. So-called "starter fertilizers" have a higher percentage of phosphorus (the middle number in the N-P-K analysis) to encourage root growth. In all cases, these soluble fertilizers are mixed with water according to label directions. Dilute one part compost tea with three parts water.

Members of the cabbage family often show red to purple coloration on leaves and stems. This is sometimes exaggerated when plants are stressed, especially transplants that have remained in the same container for more than 5-6 weeks.

Cold Damage

Cold damage is frequently observed on young tomato plants in early spring. Healthy plants out-grow the injury.

phosphorus deficiency young tomato plant
Phosphorus deficiency on young tomato leaf

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